A wide variety of articles are available that have been packaged in a heat shrink packaging film. Heat shrink packaging films offer the advantage of being relatively inexpensive, yet flexible for packaging a great variety of articles in a range of shapes and sizes. When heat shrink packaging films are used, the film is first wrapped about an article so that the article is substantially enclosed within the packaging film, whereupon an end seal is formed at each of the spaced ends of the article. This is typically done on a high speed continuous motion packaging or wrapping machine constructed to wrap the articles in the packaging film. Once the end seals are formed in the heat shrink film, the wrapped package is passed to a downstream heat shrink tunnel, in which a quick blast of heated air, for example, is directed against the film wrapped article to shrink the packaging film tightly about the exterior of the article, thus providing a neat, finished package in which the article is protected by a generally impervious barrier of packaging film.
As the articles to be packaged move along a path of travel on the wrapping machine, therefore, a continuous tube of the packaging film is formed about the articles so that the articles are generally enclosed within a tube of the film, and spaced from one another within the tube of film as the articles are thereafter carried with the film toward a downstream sealing and crimping head assembly. This is typically accomplished on a horizontal wrapping machine having opposed and counter-rotating sealing and crimping heads, between which the film entrained articles are sequentially passed. In this type of wrapping machine, the upper sealing and crimping head typically includes an elongate heated knife which extends transversely across the path of travel, and is of a length sufficient to form an end seal across the respective and spaced ends of the articles being packaged. The heated knife will rotate into engagement with a counterrotating spring loaded silicon rubber platen carried on the opposed sealing and crimping head, and which is sized to receive the knife. As the packaging film is passed between the counterrotating sealing and crimping heads, therefore, it is pinched by the knife against the platen, whereupon the heat of the knife acts to melt or otherwise form the end seal at the ends of the article. As the article is being packaged, the packaging film, and the counter-rotating heads all move in the direction of the path of travel extending from an upstream end of the machine toward a downstream end thereof.
As the heated knife moves out of engagement with the newly formed end seal of the now "wrapped" article, there is a tendency for the packaging film to adhere to the heated surfaces of the knife. This has necessitated the use of spring loaded clamp bars mounted on the rotating shaft on which the knife is mounted, which are positioned with respect to the knife, for holding the packaging film against the platen and away from the sides of the knife during seal formation, and for also stripping the packaging film off of the knife edge and/or side surfaces of the knife after the end seal has been formed and as the article moves downstream away from the counterrotating sealing and crimping heads.
A typical wrapping machine of this type will also use a pair of spaced clamp bars, positioned on opposite sides of each knife or knives provided. So constructed, the clamp bars extend parallel to, and run along the length of each knife. As the knives are rotated through an arc toward and into engagement with the packaging film, i.e. when the knife moves the packaging film against the platen, the clamp bars move into engagement with the packaging film and act to hold it away from the sides of the knife so that the film will not otherwise melt onto or adhere to the sides of the knife, and hopefully this allows only the knife edge to come into contact with the packaging film for forming the end seal. Moreover, to further prevent the packaging film from adhering to the sides of the knife, as well as to the clamp bars, oftentimes the clamp bars are water cooled which thus necessitates a rather complicated construction of a water cooling system adapted for supplying the rotating clamp bars with water, with the resultant problems of component wear due to the high demands placed on packaging machines in high speed packaging operations.
One problem that has arisen due to the construction of these conventional knife and clamp bar assemblies is that a sufficient amount, i.e., length, of packaging film is required at the ends of the article so that as the clamp bars hold the film away from the knife and against the platen, enough slack film is present to allow the knife edge to come into contact with the film against the platen, which can result in surplus film being present at the ends of the wrapped article. If, for example, a relatively low energy shrink film is used, this can result in an incomplete shrinking of the film about the ends of the article after being passed through the shrink tunnel, thus resulting in what is known to those in the art as "dog-ears" or "fish-eyes" at the ends of the package after heat shrinking the film. Aside from being an unsightly problem, this also leads to the possibility that the packaging film may be caught or torn during further processing and handling, which may result in breaking the seal of the film about the package, which, of course, leads to lowered production efficiencies, as well as leading to increased problems for retailers who may have articles that are inadvertently torn or opened while on display.
Another problem with the known sealing and crimping assemblies using knives and clamp bars is that the end seal may sometimes be incompletely formed due to the failure of the film to melt together into an integral seal because the end seal is not sufficiently cooled to allow the film forming the end seal to remain sealed, i.e. melted together, thus resulting in an opening being formed at the ends of the article, which opening may be opened moreso, or created in the first instance, by the passage of the article through the shrink tunnel. If the end seal is not adequately sealed in the first instance, the quick blast of heated air which shrinks the film about the article in the shrink tunnel may act to open the unformed seal at the ends of the article rather than draw the film tightly about the ends of the article to complete the packaging operation. It is therefore desirable to cool the newly formed end seals as quickly as possible in order to complete formation of the end seal prior to passage of the article into the heat shrink tunnel. However, this is problematic with the known knife and clamp bar arrangements.
Yet, another problem that exists with the known knife and clamp bar arrangements is that the packaging film will invariably come into contact with the sides of the knife, adjacent the knife edge, with the result that plastic residue will build up on the knife, thus necessitating machine shutdown so that the knife edge and sides can be stripped of the melted plastic residue that builds upon these surfaces. The clamp bars may also need to be cleaned of this plastic residue.
What is needed, therefore, but seemingly unavailable in the art, is an improved end seal wrapping machine which dispenses with the use of clamp bars for holding the packaging film with respect to the sealing knife, and which also is adapted to strip any packaging film which comes into engagement with the knife off of the knife. What is also needed is an improved end seal and wrapping machine which will ensure that a completed end seal is formed in the film wrapped about the ends of the article prior to passing the article through a heat shrink tunnel, such that the ends of the package are not otherwise opened, or left with surplus film after the packaging film is shrunk about the article. Additionally, there is a need for such an improved end seal wrapping machine which provides for improved production rates, and thus improved operating efficiencies and lower packaging costs, in a high speed continuous motion packaging operation. Moreover, such an improved end seal wrapping machine should cool the packaging film at the end seals as much as possible, without the use of any kind of a water cooled clamp bar, to permit these higher production rates to be attained.